The White House has apologized to the British government after alleging that a UK intelligence agency spied on President Donald Trump at the behest of former President Barack Obama.

National security adviser H.R. McMaster spoke with his British counterpart on Thursday about press secretary Sean Spicer's comment from the White House podium about a Fox News report that said British intelligence helped wiretap Trump Tower during the 2016 campaign, a White House official said Friday.

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The official described the conversation as "cordial" where McMaster described Spicer's comment as "unintentional."

McMaster also told his counterpart that "their concerns were understood and heard and it would be relayed to the White House."

The official said there was "at least two calls" from British officials on Thursday and that the British ambassador to the United States called Spicer to discuss the comment.

"Sean was pointing to the breadth of reporting, not endorsing any specific story," the official said.

A senior administration official told CNN that Spicer and McMaster offered what amounted to an apology to the British government.

Earlier Friday, a spokesman for British Prime Minister Theresa May said senior UK officials had protested to the Trump administration after the claims were repeated by Spicer.

"We've made clear to the US administration that these claims are ridiculous and should be ignored. We've received assurances that these allegations won't be repeated," May's spokesman said.

At a Thursday press briefing, Spicer read out allegations originally made on Tuesday on Fox News by legal analyst Andrew Napolitano, that the UK intelligence agency GCHQ -- the equivalent of the US National Security Agency -- had spied on Trump.

"Judge Andrew Napolitano made the following statement, quote, 'Three intelligence sources have informed Fox News that President Obama went outside the chain of command (to spy on Trump). He didn't use the NSA, he didn't use the CIA ... he used GCHQ,'" Spicer told journalists.

The strong language from Downing Street -- which followed a similar, rare statement from the UK intelligence agency GCHQ -- indicated that the British government was furious that the US had made such an incendiary allegation.